Cyclic peptides are basis of many biologically active molecules and examples are found in hormones, toxins, antibiotics and alkaloids. They also have great implications in recent studies of their linear counter parts, the proteins. They also serve as excellent model compounds for the study of protein conformation under controlled and stringent conditions and eliminating end effects. Cyclic peptides may occur with varying ring sizes and also may contain non-protein amino acids like N-methyl and D-amino acids. In this project we plan to study the detailed structure and conformation of a few important selected cyclic peptides by x-ray crystallographic method.